1. Field of the Invention
This invention relates to an apparatus for positively suppressing vibrations of building structures caused by such external forces as earthquakes and winds.
2. Description of the prior art
The present inventors have been invented an apparatus for positively suppressing vibrations of a building, which comprises an additional mass and an actuator and is provided on the top or the like of the building to apply, when the building experiences external forces of an earthquake or winds, a force tending to suppress the vibrations to the building from the reaction force of a weight as the additional mass produced through control of the operation of the actuator (as disclosed in Japanese Patent Laid-open Publications Nos. 62-268478 and 63-78974).
FIG. 1 illustrates the outline of the positive vibration suppression apparatus. On the top of a building 1, for example, a weight 2 is provided as an additional mass such that it is substantially separated from the building 1, and an actuator 3 is interposed between the weight 2 and a portion of the building 1. When vibrations of the building 1 are produced by an earthquake or winds, a sensor 4a provided in the building 1 senses the vibrations and provides a signal to a control circuit, which in turn supplies an output signal corresponding to the vibrations to a servo valve connected to the actuator 3 to control the actuator 3. Further, another sensor 4b may be provided on the side of the actuator 3 to effect feedback control of the movement of the actuator 3. While the above control is based on a closed loop control, it is also possible to measure a response of the building through analysis of earthquake waves supplied from wide and narrow bandwidth seismometers and combine the result of the analysis with an opened loop control.
However, a delay with respect to the signal in the operation time is produced in a mechanical part of the apparatus. Such a time delay should be reduced.
Further, earthquakes and winds are natural phenomena, and their scales can not be forecast when designing the apparatus. Therefore, where the maximum performance of the apparatus is determined with respect to frequently occurring medium scale earthquakes and typhoons with wind velocities of 15 m/sec. or less, it is necessary to prevent an excessive load above the capacity of the apparatus from being applied to the apparatus at the time of occurrence of a large scale earthquake. Thus, it is necessary to suppress a control force with respect to vibrations when large scale vibrations of the building take place.
Further, nobody can tell when an earthquake takes place, and the apparatus should be rendered operative as soon as an earthquake occurs. Further, it takes time until a steady operation state of an oil hydraulic system sets in after the start. This means that it is necessary to have the system warmed up by starting it to be ready for the occurrence of an earthquake at all time. For this reason, the system requires extreme running expenditures such as electric power expenditures. In addition, the life of the system is not so long.
Further, strong winds do not appear at all time, so that a large capacity electric motor need not be held operative at all time.
Further, the weight 2, which is made of steel with a mass of about 1/100 of the weight of the building, should be such that a force of the actuator 3 is smoothly transmitted to the weight. Further, the installation space is desirably as small as possible.
Further, the actuator 3 desirably operates at a comparatively high speed and has a large capacity, and is desirably constituted by an oil hydraulic cylinder or the like to realize a large stroke apparatus. In this case, it is important to secure the oil hydraulic cylinder installation space inclusive of the space for the stroke.
Further, since the weight 2 has a considerably large mass, it is necessary to reduce the influence of frictional forces as much as possible to obtain efficient and reliable vibration suppression.
Where an oil hydraulic cylinder is used for the actuator 3, the direction of control is limited, that is, it is impossible to suppress the vibrations of the building in certain directions although the vibrations in other directions can be suppressed.
Depending on the design of the building, it is usually possible to obtain a comparatively large effect through control only in one direction. However, externally applied forces due to earthquakes and winds are uncertain forces, and, in an eccentric building or the like, further effective vibration suppression can be obtained by suppressing the torsional component of vibrations.
Further, in high story or ultra-high story buildings, the secondary vibration component is often large. Therefore, it is possible to improve the vibration suppression effect by suppressing the secondary vibration component.